I just practice offline for improving my cornering and lap times.
I do my passing practice online live players is more fun.
If you want to practice passing cars try starting from the back of the start line.
I start dead last every race and usually finish in the top 5 depending on how many fast drivers are in the race.
If you hang back at start most of the bad drivers will crash at the first corner or by the 2nd corner then work on catching the others you wont catch the 1:12 drivers like that unless they crash but it's good practice.
To get dead last start position nearly everytime spectate on race restart countdown wait till race is restarting screen appears then Shift +p then join real quick. won't work if race is full.
If your super fast try to get a front position and follow the fastest cars in the race.
If you get behind that slow guy who's blocking or trying to run you off the road then i usually ride on there bumper and give them a speed boost into a wall or something